If in a language phonemic stress co-exists with phonemic quantity, one of the two elements is subordinate to the other, and three, almost never four, distinct entities are admitted: either long and short vowels are distinguished only in the stressed syllable or only one of the two quantitative categories, length or brevity, may carry a free, distinctive stress.
Standardized
Provided both stress and quantity are distinctive, either stress distinctions will be limited in terms of quantity (to either long or short vowels) or quantity distinctions will be limited in terms of stress (to stressed syllables only).
Cf. Hyman’s reformulation: Languages where both length and stress appear as distinctive features are quite exceptional.